The present invention generally relates to electronic apparatuses and more particularly to a heat sink structure adapted for mounting on a substrate of an electronic apparatus for cooling the same.
With recent developments in high speed computers and other electronic processing apparatuses, there is a demand for increasing the mounting density of module substrates that are used for carrying a circuit module. However, such an increased mounting density has led to a problem of increased heat generation. Therefore, there is an acute demand for a more efficient heat sink structure characterized by a reduced thermal resistance for heat dissipation, for module substrates.
Generally, heat sinks of module substrates have a size corresponding to the size of the module substrate itself and may have a size as large as 70 mm for each edge. Such a heat sink having a large size and designed for mounting directly upon the module substrate, is required to have a small rigidity such that the difference in the thermal expansion between the heat sink and the substrate is effectively absorbed. In recent electronic apparatuses, as a result of the increased mounting density of the substrates or mother boards within the electronic apparatuses, there is a tendency that the substrates are disposed relatively to each other with a decreased separation.
When air cooling an electronic apparatus having such a structure by air, it is preferable to apply the cooling air so as to flow generally parallel to the substrates rather than applying the cooling air perpendicularly to the substrate. Thereby, one can simplify the construction of the apparatus and reduce the space needed for cooling.
Therefore, it has been desired to provide a heat sink that is: (1) applicable to the cooling system wherein the cooling air flows parallel to the circuit substrate; (2) characterized by a small thermal resistance; and (3) characterized by a small rigidity.
FIGS. 1(A) and 1(B) show an example of a conventional heat sink 10 that is disclosed in the Japanese Laid-open Patent Publication 59-202657.
Referring to the drawings, the heat sink 10 includes a number of cooling fin elements 11 connected with each other in X- and Y-directions by a base 12 to form a matrix, wherein each fin element 11 has a top surface formed with a hole 13.
Each of the cooling fin elements 11 is formed with a cutout 14 that penetrates into the element 11 in the X-direction and another cutout 15 that penetrates into the element 11 in the Y-direction. Further, there is formed a gap between adjacent cooling fin elements 11 wherein, in correspondence to the gap, there are formed grooves 16 and 17 such that the groove 16 extends in the X-direction parallel to the cutout 14 and the groove 17 extends in the Y-direction parallel to the cutout 15. The heat sink 10 is mounted on a module substrate by soldering the base 12 thereon.
The heat sink 10 of FIGS. 1(A) and 1(B) is cooled by applying a flow, or jet, cooling air 20 vertically from the upward direction. Thus, the construction of FIG. 1 requires a duct structure immediately above the heat sink 10 for directing the cooling air, such a duct structure increases the size, particularly the height of the apparatus. Therefore, the construction of FIGS. 1(A) and 1(B) has an obvious drawback in that one cannot increase the mounting density of the substrate boards within an electronic apparatus.
FIG. 2 shows another heat sink 30 disclosed in the Japanese Laid-open Patent Publication 56-122149.
Referring to FIG. 2, the heat sink 30 includes a heat conduction base 31, and a plurality of fins 32 are provided vertically on the base 31 with a separation from each other. Each fin 32 is formed with a number of slits 33 with a separation from each other in the X-direction by a predetermined interval, wherein the slits are formed to extend in the Y-direction over the entire length of the heat conduction base 31. Further, a passage 34 of cooling air is formed between the adjacent fins 32. The heat sink 30 is mounted upon the module substrate by soldering the heat conduction base 31.
In the case of the heat sink 30 of FIG. 2, a cooling air flow 50 is applied in the X-direction parallel to the module substrate (of which illustration is omitted in the drawing). As a result of such a construction, one can eliminate the duct structure to direct the air. Thus, the construction of FIG. 2 is suitable for increasing the mounting density of the module substrates within the electronic apparatus.
On the other hand, the construction of FIG. 2 still has a drawback in that one cannot satisfactorily reduce the thermal resistance R between the heat sink and the air. Generally, the thermal resistance R of a heat sink for dissipating the heat of a module substrate to the air is represented as EQU R=t/(.multidot.A)+1/(h.multidot.A.sub.f) (1)
wherein represents the thermal conductivity of the heat sink, A represents the area of the heat conduction base of the heat sink, t represents the thickness of the heat conduction base, h represents the heat transfer coefficient on the fin surface for dissipating heat to the air, and A.sub.f represents the surface area of the fin that is contacted by the cooling air.
FIG. 3 shows the heat dissipation caused by a heat sink 40 schematically.
It will be noted that the heat sink 40, includes a fin part 41 for dissipating heat to the air and a heat conduction base 43 for conducting the heat from a module substrate 42 to the fin 41. As indicated by an arrow 45, the heat generated at the module substrate 42 is conducted to the fin 41 via the heat conduction base 43 and the heat is transferred to the air from the fin 41 as indicated by an arrow 46.
In the foregoing Eq.(1), the first term, t/(.multidot.A), represents the thermal resistance Ra that is encountered when conducting the heat of the module substrate 42 to the fin 41 via the heat conduction base 43. On the other hand, the second term 1/(h.multidot.A.sub.f) represents the thermal resistance Rb that is encountered when dissipating heat as in FIG. 3 from the fin 41 to the air.
Returning to FIG. 2 again, it will be noted that the passage 34 of the air has a U-shaped cross section characterized by an open top 35. Thus, the passage 34 communicates to the part above the heat sink 30 where the resistance to the air flow is minimum. On the other hand, the cooling air that has entered into the passage 34 experiences a resistance caused by the friction at the surface of the fin 32, and the speed of the air decreases gradually due to the pressure loss occurring in the passage 34. Thus, when the heat sink 30 is formed to have a size of more than 70 mm in the X-direction, the cooling air cannot pass the full length of the passage 34 and, instead, escapes in the upward direction as indicated by an arrow 51. Thus, the part of the heat sink 30 that is located downstream of the point where the escape of the cooling air occurs, is no longer effectively cooled by the air flow, and the efficiency of cooling is degraded significantly.
FIG. 4 explains the foregoing phenomenon.
Referring to FIG. 4, it will be noted that the area of a fin 32 that is cooled by the air is designated as S.sub.1 and S.sub.2, while S.sub.3 and S.sub.4 represent the total area of the fins 32. As will be noted, the area S.sub.1 or S.sub.2 that is actually cooled by the air is substantially smaller than the total area S.sub.3 or S.sub.4. In a typical example, the proportion of the area S.sub.1 or S.sub.2 with respect to the area S.sub.3 or S.sub.4 is about 60%. This indicates that the effective area A.sub.f of Eq.(1) is about 60% of the actual surface area of the fin. Thus, it has been difficult to reduce the thermal resistance R in the conventional heat sink structure.
The heat sink structure 30 of FIG. 2 has another drawback in that the structure cannot absorb the thermal deformation caused particularly in the Y-direction. It should be noted that the heat sink structure 30 has a reduced rigidity with respect to the deformation in the X-direction due to the formation of the number of slits 33. As a result, the thermal stress between the heat sink 30 and the module substrate is successfully suppressed with respect to the X-direction. In the Y-direction, on the other hand, the heat sink 30 has a high rigidity and the difference in the thermal expansion between the heat sink 30 and the module substrate directly leads to a thermal stress that acts upon the interface wherein the heat sink 30 is bonded upon the module substrate. In a typical example wherein aluminum is used for the heat sink, the heat sink has a thermal expansion coefficient of about 24.times.10.sup.-6 /.degree.C. On the other hand, a typical ceramic module substrate has a thermal expansion coefficient of 4-7.times.10.sup.-6 /.degree.C. Thus, it will be noted that the thermal expansion coefficient of the heat sink is four-six times larger than that of the ceramic substrate. Such a large difference in the thermal expansion inevitably causes a large thermal stress acting at the interface between the heat sink and the substrate as mentioned above, and there is a substantial risk that the bonding of the heat sink may come off or the module substrate is cracked. Such a tendency appears conspicuous with increasing performance of the electronic apparatuses and increasing heat generation of the module substrate. Further, such a tendency is enhanced with a increasing size of the module substrate.